The government must obtain a court warrant to require internet service providers to turn over stored e-mail to the authorities, a federal appeals court ruled Tuesday.

The decision by the 6th U.S. Circuit Court of Appeals was the first time an appellate court said Americans had that Fourth Amendment protection.

"The government may not compel a commercial ISP to turn over the contents of a subscriber's e-mails without first obtaining a warrant based on probable cause" (.pdf), the appeals court ruled. The decision—one stop short of the Supreme Court—covers Kentucky, Michigan, Ohio and Tennessee.

"I expect e-mail providers across the country will comply with this," he said in a telephone interview.

The legal brouhaha centered on Steven Warshak, founder of an Ohio herbal-supplement company that marketed male-enhancement tablets. As part of a fraud investigation, the government obtained thousands of his e-mails from his ISP without a warrant.

He appealed his 25-year conviction on those and other grounds, and the circuit court tossed his sentence on issues unrelated to the court's language concerning e-mail privacy.

"The Fourth Amendment must keep pace with the inexorable march of technological progress, or its guarantees will wither and perish," the court ruled.

The Stored Communications Act was enacted at a time when e-mail generally wasn't stored on servers at all, but instead passed through them briefly on their way to the recipient's inbox. In today's reality, e-mail can, and is, being stored on servers forever. A consortium of businesses, including Google and Microsoft, recently asked Congress to update the law and require probable cause to obtain any e-mail.